1. Field of the Invention
The present invention concerns voltage-regulated supplies, especially supplies for microwave amplifier tubes which require both a cathode supply and a collector supply, as is the case, for example, with TWTs (travelling wave tubes).
In these tubes a microwave, in order to be amplified, is made to interact with an electron beam.
This electron beam is accelerated by a very high cathode voltage, which is typically of the order of 40 to 50 kV. At the end of its interaction with the microwave, the beam still has high kinetic energy which may be:
either dissipated by the heating of a collector electrode connected to the ground and struck by the beam after interaction (this is typical in klystrons); PA1 or recovered in making the beam cross a potential barrier or "depression" created by taking the collector electrode to a given potential (this is typical in TWTs). This collector potential is not zero (otherwise we return to the previous case) but is smaller than the cathode potential. PA1 firstly, a very high voltage supply, but one with low power (for example a 40 kV/1200 W supply) with the task of achieving a biasing, with respect to the ground, of the cathode at the potential enabling the electron beam to be accelerated to make it interact with the microwave. It suffices for this supply to be a relatively low power supply, for its essential role is to bias the cathode with respect to the ground without a great flow of current between the cathode and the ground: for, the major part of the electron beam moves between the cathode and the collector and, between the cathode and the ground, there is a flow of only a relatively low current which is a leakage current due to the imperfections of the tube (this leakage current between the cathode and the ground generally does not exceed 20% of the total current). PA1 secondly, a high voltage supply, but one with high power (for example 30 kV/5000 W) designed to put the collector into depression by biasing it with respect to the cathode. Since it is desired to "pump out" the major portion of the electron charges recovered at this collector to make them move again towards the cathode, this second supply should give a relatively high current, whence its relatively high power. PA1 the cathode biasing means has one of its terminals connected to the collector and the other to the ground, and PA1 the collector biasing means are series mounted with the cathode biasing means and interposed between these means and the cathode so that the cathode voltage is equal to the sum of the voltages delivered respectively by the collector biasing means and the cathode biasing means. PA1 the cathode corresponds to an output node and the collector to a midpoint; PA1 the collector biasing means correspond to a first source, working essentially as a voltage source which is weakly regulated or unregulated and weakly filtered or unfiltered; PA1 the cathode biasing means correspond to a second source, working essentially as a current source, which is highly regulated and highly filtered but has a voltage notably lower than the voltage of the output node and has power notably lower than the output power.
In the latter case, the tube, which is called a "depressed collector" tube requires two distinct supplies:
2. Description of the Prior Art
Until now, it has been common to use only one supply unit for the high collector voltage and the very high cathode voltage, the two voltages being obtained from two stages of one and the same voltage generator.
However, an approach such as this requires supplies of very high quality as regards both the residual voltage noise and the stability of the potential delivered.
For, while the collector has little sensitivity to the voltage noise (notably the mains noise resulting from an incomplete filtering), since its only role is to make the unused charges of the electron beam flow again, by contrast the cathode supply calls for a very high voltage that is extremely stable and extremely well filtered (typically a noise smaller by -30 dB than the noise permissible on the collector) so as not to degrade the quality of the microwave amplified by the tube.
As for the stability required, it is of the order of 1% of the cathode potential. By contrast, it suffices to apply a voltage of approximately zero to one-third of the cathode voltage to the collector to obtain satisfactory operation, the sole drawback of an excessively low collector voltage being an increase in the heating of the collector (if the supply to this collector is insufficient, it will not be possible to recover all the kinetic energy of the beam, and hence a portion of this energy will be discharged as heat).